(1) Field of the Invention
The present invention relates to magnetic field sensors, and more specifically to a method that with the appropriate auxiliary electronic circuitry can compensate the effect of the earth's magnetic field on Giant Magneto-Impedance magnetic sensors.
(2) Description of the Prior Art
If a soft magnetic conductor is driven by a high-frequency (typically radio frequency) current, and then undergoes an applied changing magnetic field, the resultant electrical impedance change of the conductor (which is often significant) is called the Giant Magneto-Impedance effect. Small diameter amorphous metal wires (called fibers, because of their small cross-sectional area) give the strongest Giant Magneto-Impedance effect.
Extremely sensitive magnetic sensors can be constructed with these fibers and they are presently being used (for example) for micro-magnetic recording heads, magnetic anomaly detection, robotic and automotive control, and industrial and environmental measurement. To be effective, however, magnetic sensors need to compensate for the earth's residual magnetic field, (also referred to as the geo-field or static geo-field), which can be as large as +/−60,000 nanoTesla (nT). If a sensor is trying to measure only 1 nT or less, then it is obvious that the electronics of the sensor will “use up” the vast majority of its dynamic range responding to the large static geo-field, with little dynamic headroom left to measure much smaller, dynamic fields.
Referring to FIG. 1 there is illustrated a typical magnetic sensor 10 using a Giant Magneto-Impedance fiber 12. This sensor design represents one approach of several that is useful in displaying a magnetic field reading based on the magnetic field impressed upon the fiber 12. As is shown, there is a Giant Magneto-Impedance fiber 12 connected to a radio frequency signal source 14. The magnetic field (H) is impressed upon the fiber 12, as is the field from the bias coil 16 that is used to bias the fiber response in the linear, most sensitive region. The fiber 12 is connected to the input of an amplifier 18 and filter circuit 20 that has an output voltage that depends on the fiber's impedance. This is often implemented with operational amplifiers. At the output is an analog to digital converter 22 which digitizes the output voltage of the gain stage. The circuit design of magnetic sensor 10 illustrates that voltage is allowed to swing the entire range of the earth's geo-field (+/−60,000 nT) and illustrates the penalty in dynamic range that occurs from allowing the geo-field to overwhelm the sensor.
The analog to digital converter 22 has twenty bits of resolution, but the least significant bit represents 100 pT, or 0.1 nanoTesla, which will almost certainly get overwhelmed by system noise and will not be resolvable. The least significant bit will toggle between “0” and “1” and will not be usable. As a rule of thumb, the three least significant bits in an analog to digital converter can often be regarded as “down in the noise.” It is generally desirable (if at all possible) to place the dynamic range of a measured signal into the upper half of the analog to digital converter range to reduce quantization noise effects. This technique has been used in “floating point analog to digital converters” and is mentioned here as an illustration to show the dynamic range problems created by the static geo-field.
There are other kinds of magnetic sensors designs in addition to sensor 10 illustrated in FIG. 1. Many of these magnetic sensor designs use a Giant Magneto-Impedance fiber to control the resonant frequency of an oscillator. The oscillator must be stable enough to resolve the magnetic field measurement without drifting so much that any magnetic change to be measured (“Delta H”) is overwhelmed by the drift itself (See Hagerty, U.S. Pat. No. 7,405,559, Low-Power GMI Magnetic Detector That Utilizes a Crystal-Controlled Oscillator”). For any magnetic sensor, however, the same problem (i.e., dynamic range saturation) arises when confronted by the large static geo-field.
Referring to FIG. 2 there is illustrated an alternate magnetic sensor design 30 that includes geo-field nulling coils 34 for each axis fiber 40 (one each for the three axes) on the circuit board that are used to null out the earth's residual magnetic field. These geo-field nulling coils 34 are used instead of providing a separate Helmholtz coil, which would increase the size of the sensor assembly to approximately 2″ in diameter. Each geo-field nulling coil 34 is driven by a digital to analog converter 36. The digital to analog converter 36 receives instructions from the microcontroller 38. Although nulling the earth's residual magnetic field is achieved through this design, there is a penalty. The penalty is a large, 3 dimensional magnetic device (about the size of a small sugar cube) wrapped around each axis fiber 40, putting a serious limitation on the amount of miniaturization possible with such a sensor. Manufacturing such a geo-field nulling coil 34 would be labor-intensive and expensive.
Where situations exist when separate geo-field nulling coils 34 are not an option due to limitations in size and costs, then an alternative geo-field nulling method for magnetic sensors is needed based on impedance compensation rather than magnetic compensation to cancel out undesired static offsets.